To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). The multiple optical signals may be multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate wavelengths referred to as channels. Modern WDM systems have a high traffic capacity, for example, a capacity to carry 96 channels or more at 10 gigabits per second (hereinafter Gb/s) or more.
The optical fiber transmission system may include a relatively long trunk fiber segment that may be terminated at a transmitting and/or receiving trunk terminal. The optical fiber transmission system may further include one or more branching units situated along its trunk. Each branching unit (BU) may be connected to a branch fiber segment that terminates in a transmitting and/or receiving branch terminal. Each BU may include one or more optical add/drop multiplexers (OADM). Channels may be added to and/or dropped from the trunk fiber segment of the optical transmission system via the OADMs.
When information signals are transmitted over long distances, one or more amplifiers are provided to compensate for signal attenuation. The amplifiers used in some WDM systems (e.g., undersea systems) cannot easily be modified and are initially configured to support a fully loaded link (e.g., 96 channels, each channel carrying 10 Gb/s). In general, it may be desirable that the power per channel be sufficient to provide an adequate signal-to-noise ratio in the presence of the amplified spontaneous emission (ASE) noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers may thus be configured to provide an optical output signal at a nominal total optical power.
The nominal amplifier output power level may be insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the total amplifier output power may change very little around the nominal output power level. As additional channels are added, e.g. at a branching unit, the optical output power per channel may decrease. As channels are dropped, the optical output power per channel may increase.
Optical signals, while propagating through optical fibers, can experience nonlinear interaction. At sufficiently high values of optical power (e.g., more than 1 mW per channel), the optical signal may experience more distortion than at low optical powers (e.g., less than 1.0 mW per channel) which results in transmission penalty. Therefore, when channels are dropped, the value of optical channel power may increase, and network communication performance may suffer. Partial channel loading of a chain of optical amplifiers may result in undesirable noise accumulation in parts of the transmission band and gain reshaping effects that also degrade channel performance. In a partially loaded system, therefore, the transmission band may not be uniformly loaded with information signals and thus dummy tones or noise are added to control channel power along an optical path and provide uniform loading. Loading with dummy tones or noise may thus reduce nonlinear effects in optical fiber, avoid unnecessary gain tilt, avoid excessive noise accumulation, and avoid gain shaping effects such as Spectral Hole Burning (SHB).
Such uniform loading may occur in a branched optical communication system but adding and dropping of channels at the branching units presents unique challenges. Uniform loading may be maintained in the branch fiber segment if the signals carried on all of the trunk channels on a trunk fiber segment are passed on or dropped to the branch channels on a branch fiber segment and transmitted to a branch terminal. The signals carried on some of the trunk channels (also referred to as express channels), however, may not be intended for the branch terminal and should be prevented from reaching the branch terminal. On the other hand, if these signals are merely blocked or filtered out at the OADM branching unit, the transmission band in the branch segment may no longer be uniformly loaded and the channels carrying signals intended to be transmitted to the branch terminal will be penalized.